1. Field of the Invention
This invention relates generally to switching audio amplifiers, and more particularly to a method and circuit for reducing audible transients associated with start-up, shutdown, mute and other changes of operational mode in switching audio amplifiers.
2. Description of the Prior Art
A common problem when designing audio equipment is to avoid audible transients such as xe2x80x9cclicksxe2x80x9d and xe2x80x9cpopsxe2x80x9d at power-up, shutdown, mute, and the like. Transients are avoided in traditional audio amplifiers during power-up and shutdown by disconnecting the loudspeaker using a mechanical relay when transients might occur from the output stage.
When the modulation starts or stops in a pulse width modulation (PWM) amplifier, there will be certain transients with energy in the audio band, depending on the actual modulation scheme. The transients occur even though no audio signal is applied to the amplifierxe2x80x94it is simply the start of the PWM of a zero audio signal (herein after xe2x80x9cPWM zero-signalxe2x80x9d) that causes the transients. This will often result in easily audible clicks, which are not accepted in most audio applications. The traditional technique, with a relay at the output, can be used to avoid the transients; but a relay in the system has several disadvantages such as cost, required space and reliability. A second traditional approach to reduce the transients in PWM amplifiers is to start and stop the modulation only while the power supply voltage to the output stage is low. While the modulation is on, the power supply voltage is slowly ramped to or -from the desired operating value, depending on whether the modulation is to be started or stopped. This second traditional approach requires a power supply, capable of ramping the voltage up and down, which significantly increases complexity and cost.
In view of the foregoing, it would be desirable to have a method for minimizing the audio band energy in the start and stop transients associated with start-up, shut-down, mute and other changes of operational mode in switching audio amplifiers, such that the transients will be substantially inaudible.
The present invention is directed to a system and method for reducing audible turn-on and turn-off transients in switching amplifiers. The invention, according to one embodiment, employs a frequency shaped start sequence in front of a pulse width (PW) modulated zero-signal or a frequency shaped stop sequence following a PW modulated zero-signal.
According to one embodiment, the present invention comprises a method of reducing audible transients in a switching audio amplifier comprising the steps of:
providing a PWM amplifier responsive to a zero-signal delay control signal and a logic control signal to selectively start or stop modulation of a zero-signal; and
selectively starting or stopping modulation of a zero-signal in response to the zero-signal delay control signal and the logic control signal such that an output signal generated by the PWM amplifier in response to a zero-signal received by the PWM amplifier will have a substantially reduced audible transient whenever the PWM amplifier starts or stops modulating the zero-signal.
According to another embodiment, the present invention comprises a switching amplifier audible transient reduction system comprising:
a modulator;
a delay control element operational to generate a delayed zero-signal in response to a zero-signal generated by the modulator; and
a logic control element operational to cause the switching amplifier to generate an AD modulated zero-signal in response to the delayed zero-signal.
According to yet another embodiment, the present invention comprises a switching amplifier audible transient reduction system comprising:
a modulator;
a delay control element operational to generate a delayed zero-signal in response to a zero-signal generated by the modulator;
a combinational element operational to combine the delayed zero-signal with the zero-signal generated by the modulator to generate a combined zero-signal; and
a switch operational to select the delayed zero-signal or the combined zero-signal such that the logic control element can operate to selectively cause the switching amplifier to generate the AD modulated zero-signal or an ABD modulated zero-signal.
According to still another embodiment, the present invention comprises a switching amplifier audible transient reduction system comprising:
modulating means for modulating a zero-signal;
delaying means for delaying the modulated zero-signal;
combining means for combining the delayed zero-signal with the modulated zero-signal to generate a combined zero-signal;
logic signaling means for generating a logic control signal; and
switching means for selecting the delayed zero-signal or the combined zero-signal such that the logic signaling means can operate to selectively cause the switching amplifier to generate an AD modulated zero-signal or an ABD modulated zero-signal.
According to still another embodiment, the present invention comprises a switching amplifier audible transient reduction system comprising:
modulating means for generating a modulated zero-signal;
delaying means for generating a delayed zero-signal in response to the modulated zero-signal; and
logic signaling means for generating a logic control signal such that the logic signaling means can operate to cause the switching amplifier to generate an AD modulated zero-signal in response to the delayed zero-signal.
According to yet another embodiment of the present invention, a method of reducing audible audio amplifier output transients comprises the steps of:
providing an audio amplifier having a signal input and configured to implement a predetermined modulation scheme;
receiving a zero audio input signal at the signal input of the audio amplifier;
adding a frequency shaped sequence of signal pulses to the zero audio input signal; and
generating an audio amplifier output signal in response to the zero audio input signal and the frequency shaped sequence of signal pulses such that the audio amplifier output signal exhibits substantially no audible noise, wherein the frequency shaped sequence of signal pulses is predetermined by the modulation scheme.
The method may further comprise configuring the frequency shaped sequence of signal pulses such that the audio amplifier output signal exhibits substantially no audible noise prior to and/or subsequent to modulating the zero audio input signal.
Still another embodiment of the present invention comprises: A method of reducing audio switching amplifier output noise artifacts comprising the steps of:
providing a switching amplifier system comprising a pulse width modulator (PWM) operational in association with a predetermined modulation scheme and having a signal input, an output stage operational to receive input signals processed by the PWM to provide PWM output signals, and means for shaping signal transitions between switching amplifier system xe2x80x9cONxe2x80x9d and xe2x80x9cMutexe2x80x9d states associated with generation of the PWM output signals;
receiving an input signal via the PWM;
and
shaping signal transitions between switching amplifier system xe2x80x9cONxe2x80x9d and xe2x80x9cMutexe2x80x9d states to generate the PWM output signals, wherein the PWM output signals comprise a frequency shaped sequence of signal pulses such that the PWM output signals exhibit substantially less audible noise than PWM output signals generated by the switching amplifier system in the absence of signal transition shaping.
Still another embodiment of the present invention comprises: A switching amplifier system comprising:
a pulse width modulator (PWM) having a signal input; and
an output stage operational to receive input signals processed by the PWM and provide PWM output signals therefrom, wherein the PWM output signals comprise a frequency shaped sequence of signals generated at least in part via a switching amplifier system signal transition shaping means such that noise artifacts associated with the PWM output signals are substantially less audible than noise artifacts associated with PWM output signals provided by the switching amplifier system in the absence of the signal transition shaping means.
In one aspect of the invention, a system and method functions to minimize audio band energy in the start and stop transients associated with start-up, shut-down, mute and other changes of operational mode in switching audio amplifiers, such that the transients will be inaudible in practice.
As used herein, amplifier xe2x80x9cONxe2x80x9d state means the normal operating state of the amplifier, e.g. when playing audio or zero-audio (silence). The output stage is switching in the xe2x80x9cONxe2x80x9d state.
As used herein, amplifier xe2x80x9cMUTExe2x80x9d state means the amplifier is silent and no switching is happening in the output stage.